a. Field of the Invention
This invention relates to a light beam scanning device for recording or reading of an image.
FIG. 1 of the accompanying drawing shows a perspective view of one embodiment of the laser beam printer as one example of a device, in which the light beam scanning optical system is utilized. In FIG. 1, a laser beam oscillated from a laser beam oscillator 1 is introduced into an input opening of a modulator 3 through reflecting mirrors 2, 2. The beam which has been subjected to modulation of an information signal to be recorded in the modulator 3 is expanded its beam diameter by a beam expander 4 maintaining its parallel beam, and is projected onto a rotatory polygonal reflecting mirror 5. The rotatory polygonal reflecting mirror 5 is fitted on a shaft which is supported on a highly precise bearing member, and is rotated by a motor 6 rotating at a constant speed. Accordingly, the beam to be deflected by the rotatory polygonal reflecting mirror 5 is deflected at a constant angular speed. The beam deflected by the rotatory polygonal mirror 5 is focussed on a photosensitive drum 8 by a focussing lens 7. Numerals 9 and 10 respectively designate a first corona charger and an a.c. corona discharger, both constituting parts of the electrophotographic processes. In the device of this construction, when the rotatory polygonal mirror 5 is rotated by .theta./2, the deflected quantity of the incident angle of the light beam projected into the focussing lens 7 becomes .theta.. In this case, if an f.multidot.tan .theta. lens, in which an image height is proportional to a tangent of an angle of deflection, is used as the focussing lens 7 as in the case of photographic lens in general, a moving quantity y of the beam on the photosensitive drum 8 becomes y=f.multidot.tan .theta., so that the rotational angle of the rotatory polygonal mirror is not proportional to the moving quantity of the beam on the photosensitive drum. As the consequence, even when the rotatory polygonal mirror deflects the beam at a constant angular speed, the beam on the photosensitive drum (the surface to be scanned) does not move at a constant speed. In this case, when a lens, in which the image height of the beam on the photosensitive drum is proportional to the angle of deflection of the rotatory polygonal mirror, is used for the image focussing lens 7, the image height can be represented by y=f.multidot..theta., whereby the angle of rotation of the rotatory polygonal mirror becomes proportional to the moving speed of the beam on the photosensitive drum. In other words, for the scanning beam to be focussed flat on the scanning surface at a constant speed by the use of a deflector which effect the deflection at a constant angular speed, it becomes necessary to use a lens having a characteristic of y=f.multidot..theta. (hereinafter referred to as "f-.theta. lens") as the focussing lens.
b. Description of the Prior Art
For the f-.theta. lens in the conventional light beam scanning device, there is Japanese laid-open patent application No. 51-9463, the construction of which is shown in FIG. 2.
In FIG. 2, a reference numeral 11 designates a deflector such as rotatory polygonal mirror, 12 refers to a group of scanning lenses arranged in the power order of "concave", "convex", and "convex" viewed from the side of the deflector 11. Incident parallel light beam is deflected by the deflector 11, and then projected into the lens system 12. The lens system 12 is flat on the scanning surface, and has such a characteristic that it indicates y=f.multidot..theta. of an image height proportional to the angle of deflection. In this lens system, the intended object is attained with the three-lens structure.
For the f-.theta. lens used in another light beam scanning device, there is U.S. Pat. No. 3,668,984, the detailed structure of which is shown in FIG. 3A, and its aberration diagram in FIG. 3B. In FIG. 3A, a reference numeral 13 designates a deflector such as galvano-mirror, and 14 refers to a scanning lens system consisting of four single lenses, in which the middle two are mutually put together, and the three-lens group is arranged in the order of "convex", "concave", and "convex" viewed from the side of the deflector. Incident parallel light beam is deflected by the galvano-mirror 13, and then projected into the lens system 14. The lens system 14 is flat on the scanning surface, and has the characteristic of y=f.multidot..theta. of an image height proportional to the angle of deflection. With this lens system, however, it is difficult to carry out the wide-angle scanning. That is, as is seen from FIG. 3B, when .omega./2.div.15.8.degree., the characteristic of the angle of view becomes remarkably deteriorated.
As shown in FIG. 4, when a non-afocal beam is scanned by the deflector, there arises inconveniences to be mentioned in the following. That is, the optical system shown in FIG. 4 is that taught is U.S. Pat. No. 3,946,150, in which non-parallel beam is scanned by the deflector and projected into the focussing lens. In this drawing, a reference numeral 15 designates a spherical lens, or a cylindrical lens having a power within the plane of the drawing, which focusses on a point 19. 16 refers to a reflecting surface of a deflector such as a rotatory polygonal mirror, etc., 17 denotes a focussing lens system which is flat on the scanning surface 18, and renders the moving speed of the focussed spot constant. In this optical lay-out, the parallel light beam is converged on a position 19 by the lens 15, and the dispersed light from this point is deflected by the deflector 16 to enter into the focussing lens 17, by which it is focussed on the scanning surface 18. Accordingly, the object point of the focussing lens 17 is on the point 19, hence this object point is focussed on the scanning surface, and it is a finite image focussing. From this, since the object point 19 moves on an arc 20 by the rotation of the deflector 16, the focussing lens 15 is required to render the curvature of the object point to be flat on the image forming surface, and to render the image height constant by producing a predetermined distortion. Further, since the curvature of the object surface is determined by the arrangement of the lens 15 and the deflector 16, curvature of the object surface varies from system to system. Thus, finitude in the focussing lens results in such disadvantages that difficult problem would arise on rectification of aberration in the image forming lens due to curvature of the object surface, and the curvature of the object point differs from system to system, hence the curvature correction is not common and general.